do I have to know the OS to use it, like linux FreeBDS...........etc.......?? :D

Yes, you should. Unless you get a managed dedicated server where you don't have to worry about system fixed/upgrades :)

how much does a standard managed dedicated server cost a month??

Thats best asked in the request forum and somone will give you a quote - I know how much Kiwi's company MCHost charges but im not allowed to post here :(

If you are looking for managed dedicated servers I would recommend you check out rackspace.com (http://www.rackspace.com)
They have a very good reputation and I have heard many good things about them.

I personally think you should just hire an admin.

There is no point having a managed server if you dont know your way around it.

A managed hosting company will not add your sites to your box for you, manage accounts, troubleshoot why your httpd is slightly laggy, install SSL certs for you, nor hold your hand the whole way.

Get an ADMIN!

Usually managed hosting means 'managed' hosting ...agreed, most things can be done from WHM/Plesk, etc. but i doubt an SSL installation takes up too much of any provider's time.